The present invention relates to a flow control valve. More particularly, the invention relates to a flow control valve which is able to control the quantity of oil fed to hydraulic devices from an oil pump in response to the increase in the quantity of discharged oil from the oil pump.
There are hitherto known flow control valves which are so designed that, when the quantity of oil discharged from the oil pump is increased, the quantity of oil that is fed to the hydraulic device from the oil pump is decreased. The flow control valve of this kind is basically provided with an orifice disposed midway in a flow passage which communicates the delivery opening of the oil pump with the hydraulic device, and a spool valve which is actuated in response to the hydraulic pressure difference generated in front of and behind the orifice and returns the oil, in a quantity responsive to the degree of actuation thereof, from the flow passage to the suction port of the oil pump. With an increase of the hydraulic pressure difference, the quantity of oil which is fed to the hydraulic device is decreased.
A means for increasing such hydraulic pressure difference is exemplified by a device (as disclosed in U.S. Pat. No. 3,314,495) in which the aspirator effect due to the flow of fluid is utilized to reduce the oil pressure on the low pressure side so that the hydraulic pressure difference is increased and the quantity of the feed oil is decreased, and a device (as disclosed in U.S. Pat. No. 3,426,785) in which, separately from the above orifice, there is provided a variable orifice which is composed of a spool valve and an opening of a flow passage into a hole, said flow passage permitting the hydraulic oil to pass through the hole in which the spool valve is slidably fitted. So that the opening is gradually closed directly by the spool valve, thereby throttling the flow passage and increasing the hydraulic pressure difference.
The flow control valve having such a flow characteristic is commonly used for the power steering of a vehicle and serves to provide stability of the vehicle when it runs at high speeds and a proper sensitivity in steering. Incidentally, in the case that the flow control valve of this type as described above is applied to the power steering device of a vehicle, it is so designed that in order to supply minimal hydraulic oil even at high-speed running so as to assure smooth operation of the power steering device, the quantity of oil fed to the power steering device is reduced to a certain level and after that the quantity of oil supply is maintained constant to supply minimal hydraulic oil.
In the flow control valves of the conventional art, however, the quantity of oil supply that is controlled to a given minimal value as described above is gradually increased with an increase of the quantity of discharged oil of the oil pump, which is liable to give an adverse effect to the high speed stability of the vehicle. In addition, the shaping of machine parts to obtain various flow characteristics is difficult, and the freedom in setting the flow characteristics is small due to the difficulties in the shaping and the structure. Moreover, it is impossible for the prior art flow control valves to increase freely the quantity of oil which is fed to the hydraulic device with the increase in the quantity of discharged oil of the oil pump after the quantity of oil fed to the hydraulic device is once reduced.